Some of the most remarkable adaptations in plants include adaptation to extreme edaphic stress. Therefore we are applying genome scanning approaches to understand the solutions evolution offers in some of the many independently-evolved lineages of metal, drought, and serpentine-tolerant plants. Our initial results point to broadly orchestrated, polygenic responses to these selective pressures. Concerted changes indicate that even in the case of an environmetnal pressure, well-orchestrated internal adaptations may be marshalled, calling for a better understanding of internal adaptation (i.e., systems-based analysis of compensatory changes in the genome). In addition, these first results are pointing to striking instances of repeated evolution and gene flow between species that may mediate some of these adaptations.
Serpentine soils present a multidimensional hazard to plant life. Not only do they offer marginal levels of essential nutrients such as Ca, P, K, and N, but they are also usually very porous, with a high propensity toward drought. Low Ca:Mg ratios are a defining feature of serpentine environments and result in very low Ca uptake. These insults are typically compounded by the presence of phytotoxic levels of heavy metals such as Ni, Cd, and Zn, which leads to stunting and chlorosis, along with antagonistic effects on Fe uptake. As a result, serpentine environments are characterized by minimal ecosystem productivity and high rates of endemism. Evolution has nevertheless forged populations that thrive among these stresses, which by adapting to this ‘serpentine syndrome,’ suggest solutions to challenges in crop improvement.
In contrast to our recent work on adaptation to genome duplication in the same A. arenosa system, we discovered a relatively diverse array of genes implicated in serpentine adaptation, from strong sweeps in dehydration tolerance coding loci (ERD4 below), to loci involved in sulfur transport (SULTR1;1), metal transport, and root growth. We see clear selective sweeps in many categories consistent with adaptation to ‘serpentine syndrome’: dehydration tolerance, ion transport (Ca, Mg, and K transport-related genes), stress, and root branching and growth.